Variable stopwrite threshold

ABSTRACT

A data storage system according to one embodiment includes a head, a drive mechanism for passing a medium over the head, a controller electrically coupled to the head, and logic integrated with and/or executable by the controller. The logic is configured to: update a first value based on a current position error signal sample, determine whether the first value exceeds a predetermined threshold, determine a stopwrite threshold based on the first value when the first value exceeds the predetermined threshold, determine whether the current position error signal sample exceeds the stopwrite threshold, disable writing when the current position error signal sample exceeds the stopwrite threshold, and enable writing when the current position error signal sample does not exceed the stopwrite threshold.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/440,904, filed Apr. 5, 2012, which is herein incorporated byreference.

BACKGROUND

The present invention relates to data storage systems, and moreparticularly, this invention relates to systems and methods foradjusting a stopwrite threshold when recording data on a medium.

In magnetic storage systems, data is read from and written onto magneticrecording media utilizing magnetic transducers commonly. Data is writtenon the magnetic recording media by moving a magnetic recordingtransducer to a position over the media where the data is to be stored.The magnetic recording transducer then generates a magnetic field, whichencodes the data into the magnetic media. Data is read from the media bysimilarly positioning the magnetic read transducer and then sensing themagnetic field of the magnetic media. Read and write operations may beindependently synchronized with the movement of the media to ensure thatthe data can be read from and written to the desired location on themedia.

An important and continuing goal in the data storage industry is that ofincreasing the density of data stored on a medium. For tape storagesystems, that goal has led to increasing the track and linear bitdensity on recording tape, and decreasing the thickness of the magnetictape medium. However, the development of small footprint, higherperformance tape drive systems has created various problems in thedesign of a tape head assembly for use in such systems.

In a tape drive system, magnetic tape is moved over the surface of thetape head at high speed. Usually the tape head is designed to minimizethe spacing between the head and the tape. The spacing between themagnetic head and the magnetic tape is crucial so that the recordinggaps of the transducers, which are the source of the magnetic recordingflux, are in near contact with the tape to effect writing sharptransitions, and so that the read element is in near contact with thetape to provide effective coupling of the magnetic field from the tapeto the read element.

Tape drives have conventionally used a servo system to keep thewrite/read heads in the correct lateral location on the tape. Thedifference between the correct location and actual location of the headsis referred to as position error signal (PES).

Current servo systems implement a fixed threshold such that if the PESis larger than the threshold, the writing of the heads will be stoppedto prevent overwriting of adjacent tracks. This threshold is referred toas the stopwrite (SW) threshold.

However, it is difficult to pick the appropriate SW threshold due to thedifferences in distributions of PES data for different drives and/ordifferent tapes. Another drawback is that when a particularpredetermined SW threshold is used, the drive may write the data withoutany apparent error, when actually the adjacent tracks have beenoverwritten, rendering the data therein unreadable. This result ishighly undesirable.

BRIEF SUMMARY

A method according to one embodiment includes updating a first valuebased on a current position error signal sample, determining whether thefirst value exceeds a predetermined threshold, determining a stopwritethreshold based on the first value when the first value exceeds thepredetermined threshold, determining whether the current position errorsignal sample exceeds the stopwrite threshold, disabling writing whenthe current position error signal sample exceeds the stopwritethreshold, and enabling writing when the current position error signalsample does not exceed the stopwrite threshold.

A computer program product according to one embodiment includes acomputer readable storage medium having program instructions embodiedtherewith, the program instructions executable by a controller to causethe controller to perform the foregoing method.

A data storage system according to one embodiment includes a head, adrive mechanism for passing a medium over the head, a controllerelectrically coupled to the magnetic head, and logic integrated withand/or executable by the controller, the logic being configured toperform the foregoing method.

Any of these embodiments may be implemented in a magnetic data storagesystem such as a tape drive system, which may include a magnetic head, adrive mechanism for passing a magnetic medium (e.g., recording tape)over the magnetic head, and a controller electrically coupled to themagnetic head.

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic diagram of a simplified tape drive systemaccording to one embodiment.

FIG. 2 illustrates a side view of a flat-lapped, bi-directional,two-module magnetic tape head according to one embodiment.

FIG. 2A is a tape bearing surface view taken from Line 2A of FIG. 2.

FIG. 2B is a detailed view taken from Circle 2B of FIG. 2A.

FIG. 2C is a detailed view of a partial tape bearing surface of a pairof modules.

FIG. 3 is a partial tape bearing surface view of a magnetic head havinga write-read-write configuration.

FIG. 4 is a partial tape bearing surface view of a magnetic head havinga read-write-read configuration.

FIG. 5 depicts a method according to one embodiment.

FIG. 6 is a flowchart of a method according to one embodiment.

FIG. 7 is a top down view of a data track according to one embodiment.

FIG. 8 is a graph according to one embodiment.

FIG. 9 is a graph according to one embodiment.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

The following description discloses several preferred embodiments ofmagnetic storage systems, as well as operation and/or component partsthereof.

In one general embodiment, a data storage system includes a head; adrive mechanism for passing a medium over the head; a controllerelectrically coupled to the head; logic for periodically determining astopwrite threshold based on a current position error signal sample;logic for determining whether the current position error signal sampleexceeds the stopwrite threshold; logic for disabling writing when thecurrent position error signal sample exceeds the stopwrite threshold;and logic for enabling writing when the current position error signalsample does not exceed the stopwrite threshold.

In another general embodiment, a method includes periodicallydetermining a stopwrite threshold based on a current position errorsignal sample; determining whether the current position error signalsample exceeds the stopwrite threshold; disabling writing when thecurrent position error signal sample exceeds the stopwrite threshold;and enabling writing when the current position error signal sample doesnot exceed the stopwrite threshold.

In another general embodiment, a computer program product includes acomputer readable storage medium having computer readable program codeembodied therewith, the computer readable program code comprising:computer readable program code configured to update a first value basedon a current position error signal sample; computer readable programcode configured to determine whether the first value exceeds apredetermined threshold; computer readable program code configured todetermine a stopwrite threshold based on the first value when the firstvalue exceeds the predetermined threshold; computer readable programcode configured to determine whether the current position error signalsample exceeds the stopwrite threshold; computer readable program codeconfigured to disable writing when the current position error signalsample exceeds the stopwrite threshold; and computer readable programcode configured to enable writing when the current position error signalsample does not exceed the stopwrite threshold.

In another general embodiment, a data storage system includes a head; adrive mechanism for passing a medium over the head; a controllerelectrically coupled to the magnetic head; logic for updating a firstvalue based on a current position error signal sample; logic fordetermining whether the first value exceeds a predetermined threshold;logic for determining a stopwrite threshold based on the first valuewhen the first value exceeds the predetermined threshold; logic fordetermining whether the current position error signal sample exceeds thestopwrite threshold; logic for disabling writing when the currentposition error signal sample exceeds the stopwrite threshold; and logicfor enabling writing when the current position error signal sample doesnot exceed the stopwrite threshold.

FIG. 1 illustrates a simplified tape drive 100 of a tape-based datastorage system, which may be employed in the context of the presentinvention. While one specific implementation of a tape drive is shown inFIG. 1, it should be noted that the embodiments described herein may beimplemented in the context of any type of tape drive system.

As shown, a tape supply cartridge 120 and a take-up reel 121 areprovided to support a tape 122. One or more of the reels may form partof a removable cartridge and are not necessarily part of the system 100.The tape drive, such as that illustrated in FIG. 1, may further includedrive motor(s) to drive the tape supply cartridge 120 and the take-upreel 121 to move the tape 122 over a tape head 126 of any type. Suchhead may include an array of readers, writers, or both.

Guides 125 guide the tape 122 across the tape head 126. Such tape head126 is in turn coupled to a controller assembly 128 via a cable 130. Thecontroller 128 typically controls head functions such as servofollowing, writing, reading, etc. The controller may operate under logicknown in the art, as well as any logic disclosed herein. The cable 130may include read/write circuits to transmit data to the head 126 to berecorded on the tape 122 and to receive data read by the head 126 fromthe tape 122. An actuator 132 controls position of the head 126 relativeto the tape 122.

An interface 134 may also be provided for communication between the tapedrive and a host (integral or external) to send and receive the data andfor controlling the operation of the tape drive and communicating thestatus of the tape drive to the host, all as will be understood by thoseof skill in the art.

By way of example, FIG. 2 illustrates a side view of a flat-lapped,bi-directional, two-module magnetic tape head 200 which may beimplemented in the context of the present invention. As shown, the headincludes a pair of bases 202, each equipped with a module 204, and fixedat a small angle α with respect to each other. The bases may be“U-beams” that are adhesively coupled together. Each module 204 includesa substrate 204A and a closure 204B with a thin film portion, commonlyreferred to as a “gap” in which the readers and/or writers 206 areformed. In use, a tape 208 is moved over the modules 204 along a media(tape) bearing surface 209 in the manner shown for reading and writingdata on the tape 208 using the readers and writers. The wrap angle θ ofthe tape 208 at edges going onto and exiting the flat media supportsurfaces 209 are usually between about 0.1 degree and about 5 degrees.

The substrates 204A are typically constructed of a wear resistantmaterial, such as a ceramic. The closures 204B made of the same orsimilar ceramic as the substrates 204A.

The readers and writers may be arranged in a piggyback or mergedconfiguration. An illustrative piggybacked configuration comprises a(magnetically inductive) writer transducer on top of (or below) a(magnetically shielded) reader transducer (e.g., a magnetoresistivereader, etc.), wherein the poles of the writer and the shields of thereader are generally separated. An illustrative merged configurationcomprises one reader shield in the same physical layer as one writerpole (hence, “merged”). The readers and writers may also be arranged inan interleaved configuration. Alternatively, each array of channels maybe readers or writers only. Any of these arrays may contain one or moreservo track readers for reading servo data on the medium.

FIG. 2A illustrates the tape bearing surface 209 of one of the modules204 taken from Line 2A of FIG. 2. A representative tape 208 is shown indashed lines. The module 204 is preferably long enough to be able tosupport the tape as the head steps between data bands.

In this example, the tape 208 includes 4 to 22 data bands, e.g., with 16data bands and 17 servo tracks 210, as shown in FIG. 2A on a one-halfinch wide tape 208. The data bands are defined between servo tracks 210.Each data band may include a number of data tracks, for example 512 datatracks (not shown). During read/write operations, the readers and/orwriters 206 are positioned to specific track positions within one of thedata bands. Outer readers, sometimes called servo readers, read theservo tracks 210. The servo signals are in turn used to keep the readersand/or writers 206 aligned with a particular set of tracks during theread/write operations.

FIG. 2B depicts a plurality of readers and/or writers 206 formed in agap 218 on the module 204 in Circle 2B of FIG. 2A. As shown, the arrayof readers and writers 206 includes, for example, 16 writers 214, 16readers 216 and two servo readers 212, though the number of elements mayvary. Illustrative embodiments include 8, 16, 32, 40, and 64 readersand/or writers 206 per array. A preferred embodiment includes 32 readersper array and/or 32 writers per array, where the actual number oftransducing elements could be greater, e.g., 33, 34, etc. This allowsthe tape to travel more slowly, thereby reducing speed-induced trackingand mechanical difficulties and/or execute fewer “wraps” to fill or readthe tape. While the readers and writers may be arranged in a piggybackconfiguration as shown in FIG. 2B, the readers 216 and writers 214 mayalso be arranged in an interleaved configuration. Alternatively, eacharray of readers and/or writers 206 may be readers or writers only, andthe arrays may contain one or more servo readers 212. As noted byconsidering FIGS. 2 and 2A-B together, each module 204 may include acomplementary set of readers and/or writers 206 for such things asbi-directional reading and writing, read-while-write capability,backward compatibility, etc.

FIG. 2C shows a partial tape bearing surface view of complimentarymodules of a magnetic tape head 200 according to one embodiment. In thisembodiment, each module has a plurality of read/write (R/W) pairs in apiggyback configuration formed on a common substrate 204A and anoptional electrically insulative layer 236. The writers, exemplified bythe write head 214 and the readers, exemplified by the read head 216,are aligned parallel to a direction of travel of a tape mediumthereacross to form an R/W pair, exemplified by the R/W pair 222.

Several R/W pairs 222 may be present, such as 8, 16, 32 pairs, etc. TheR/W pairs 222 as shown are linearly aligned in a direction generallyperpendicular to a direction of tape travel thereacross. However, thepairs may also be aligned diagonally, etc. Servo readers 212 arepositioned on the outside of the array of R/W pairs, the function ofwhich is well known.

Generally, the magnetic tape medium moves in either a forward or reversedirection as indicated by arrow 220. The magnetic tape medium and headassembly 200 operate in a transducing relationship in the mannerwell-known in the art. The piggybacked MR head assembly 200 includes twothin-film modules 224 and 226 of generally identical construction.

Modules 224 and 226 are joined together with a space present betweenclosures 204B thereof (partially shown) to form a single physical unitto provide read-while-write capability by activating the writer of theleading module and reader of the trailing module aligned with the writerof the leading module parallel to the direction of tape travel relativethereto. When a module 224, 226 of a piggyback head 200 is constructed,layers are formed in the gap 218 created above an electricallyconductive substrate 204A (partially shown), e.g., of AlTiC, ingenerally the following order for the R/W pairs 222: an insulating layer236, a first shield 232 typically of an iron alloy such as NiFe(permalloy), CZT or Al—Fe—Si (Sendust), a sensor 234 for sensing a datatrack on a magnetic medium, a second shield 238 typically of anickel-iron alloy (e.g., 80/20 Permalloy), first and second writer poletips 228, 230, and a coil (not shown).

The first and second writer poles 228, 230 may be fabricated from highmagnetic moment materials such as 45/55 NiFe. Note that these materialsare provided by way of example only, and other materials may be used.Additional layers such as insulation between the shields and/or poletips and an insulation layer surrounding the sensor may be present.Illustrative materials for the insulation include alumina and otheroxides, insulative polymers, etc.

The configuration of the tape head 126 according to one embodimentincludes multiple modules, preferably three or more. In awrite-read-write (W-R-W) head, outer modules for writing flank one ormore inner modules for reading. Referring to FIG. 3, depicting a W-R-Wconfiguration, the outer modules 402, 406 each include one or morearrays of writers 410. The inner module 404 of FIG. 3 includes one ormore arrays of readers 408 in a similar configuration. Variations of amulti-module head include a R-W-R head (FIG. 4), a R-R-W head, a W-W-Rhead, etc. In yet other variations, one or more of the modules may haveread/write pairs of transducers. Moreover, more than three modules maybe present. In further approaches, two outer modules may flank two ormore inner modules, e.g., in a W-R-R-W, a R-W-W-R arrangement, etc. Forsimplicity, a W-R-W head is used primarily herein to exemplifyembodiments of the present invention. One skilled in the art apprisedwith the teachings herein will appreciate how permutations of thepresent invention would apply to configurations other than a W-R-Wconfiguration.

The teachings herein may be applied to other types of data storagesystems. For example, according to a general embodiment, a data storagesystem may include a head which may be magnetic, optical, etc. or anyother type of head which would be apparent to one skilled in the artupon reading the present description. The system may additionallyinclude a drive mechanism for passing an e.g., magnetic, optical, etc.medium over the head. The data storage system may further include acontroller electrically coupled to the head.

The data storage system may also include logic according to any of theembodiments described and/or suggested herein. In one approach, thelogic may be encoded in a controller and/or other hardware, stored inmemory as software or firmware and made available to the controllerand/or other hardware, etc. and combinations thereof. Moreover, thelogic may be for performing any of the process steps recited herein.

Conventional data storage systems include a predefined stopwritethreshold and can be inaccurate for any given period of writing.Depending on the situation, the stopwrite threshold can either be overlyconstraining by only permitting writing during a low PES, therebyminimizing the capacity of the tape; or it may be overly permissive bypermitting writing during high PES samples, thus allowing adjacenttracks on the medium to be overwritten.

Embodiments of the present invention overcome the aforementioneddrawback by providing a stopwrite system that is able to adjust thestopwrite threshold to accommodate varying write conditions. Preferably,such system and/or method is able to statistically calculate the PESstandard deviation (or other derivative of a PES sample) and makechanges to the stopwrite threshold accordingly, as explained in furtherdetail below. Moreover, each system and/or method may ensure theappropriate stopwrite threshold to accommodate favorable conditions suchthat written data may be read back later.

Referring now to FIG. 5, a method 500 is depicted according to oneembodiment. As an option, the present method 500 may be implemented inconjunction with features from any other embodiment listed herein, suchas those described with reference to the other FIGS. Of course, however,such method 500 and others presented herein may be used in variousapplications and/or in permutations which may or may not be specificallydescribed in the illustrative embodiments listed herein. Further, themethod 500 presented herein may be used in any desired environment.

Referring to FIG. 5, a method 500 is depicted according to oneillustrative embodiment of a simplified process for successfullyrecording data to a medium. The method 500 includes periodicallydetermining a stopwrite threshold based on a current PES sample,including values derived therefrom. In a preferred approach, thestopwrite threshold is determined using the standard deviation of thePES sample. See operation 502, which is explained in further detailbelow. It should be noted that the period may correspond topredetermined regular intervals; irregular intervals; periods calculatedon the fly e.g., as a function of data rate, tape speed, etc.; etc.

With continued reference to FIG. 5, the method 500 also includesdetermining whether the current PES sample exceeds the stopwritethreshold. See operation 504.

In operation 506, writing is disabled when the current PES sampleexceeds the stopwrite threshold.

The method 500 additionally includes enabling writing when the currentPES sample does not exceed the stopwrite threshold. See operation 508.

According to one approach the method 500 may incorporate logic whileconducting the aforementioned operations. In one approach, the logic maybe encoded in a controller and/or other hardware, stored in memory assoftware or firmware and made available to the controller and/or otherhardware, etc. and combinations thereof.

In a preferred approach, the method 500 may be executed at intervals ofless than about 1 second, more preferably less than about 0.01 seconds,still more preferably less than about 1 millisecond, but could beshorter or longer based on the desired embodiment. According to oneillustrative embodiment which is by no means meant to limit the scope ofthe invention, the aforementioned logic may be executed at regular orirregular intervals of about 50 μs.

Referring now to FIG. 6 a method 600 is depicted according to oneillustrative embodiment. As an option, the present method 600 may beimplemented in conjunction with features from any other embodimentlisted herein, such as those described with reference to the other FIGS.Of course, however, such method 600 and others presented herein may beused in various applications and/or in permutations which may or may notbe specifically described in the illustrative embodiments listed herein.Further, the method 600 presented herein may be used in any desiredenvironment.

In a preferred approach, the method 600 may be executed at regular orirregular intervals as the track is being written to.

Operation 602 includes measuring the current PES sample. In oneapproach, the previous PES samples may be measured to find thecorresponding deviation. According to one approach, the PES may bemeasured by incorporating any method known in the art, e.g., usingservos, etc.

Operation 604 includes updating a first value (e.g., σ_(raw)), which maybe based on PES samples, a current PES sample, etc. According to variousapproaches, σ_(raw) may be a standard deviation of PES samples,including prior PES samples, the current PES sample, etc. Moreover,σ_(raw) may be calculated by incorporating any formula known in the art.

In a preferred illustrative embodiment, the σ_(raw) may be calculatedusing Equation 1, where σ_(k) is used as σ_(raw).

σ_(k) ² =B×σ _(k-1) ²+(1−B)×x _(k) ²  Equation 1

According to the preferred embodiment, σ_(k) ² represents the varianceat the current PES sample, σ_(k-1) ² represents the variance at theprevious PES sample, and x_(k) represents the current PES sample. Byincorporating the variance of the previous PES samples, the accumulativedistribution may be accurate, thereby preferably resulting in anaccurate stopwrite threshold as well, without having to store all of theprevious PES sample values.

In one approach, if Equation 1 is incorporated for a first time, thevalue for σ_(k-1) ² (variance of the previous PES sample) may implementstored data from the previous PES sample, an arbitrary value chosen by auser, etc. Without wishing to be bound by any theory, it is believedthat the σ_(k-1) ² value implemented for a first time Equation 1 isused, may not significantly affect the σ_(raw) value that is calculatedand used to set the SW threshold during writing.

Depending on the embodiment, the value of B may determine how much of aneffect the previous PES sample has compared to the current PES sample,on the value of σ_(raw) being calculated. Without wishing to be bound byany theory, it is believed that a value for B between about 0.99 andabout 0.999 results in an optimal effect for most embodiments, but maybe any value.

σ_(raw) can be calculated by taking the square root of σ_(k) ².

With continued reference to FIG. 6, operation 606 includes determiningwhether the first value (e.g., σ_(raw)) exceeds a predeterminedthreshold (e.g., σ_(max)).

According to various approaches, a predetermined threshold (e.g.,σ_(max)) may be calculated using any method known in the art; however anillustrative example, which is in no way meant to limit the invention,is provided.

In the following example, assume a data storage system includes amagnetic tape head writing data to shingled data tracks on the tape ofthe magnetic tape head as shown in FIG. 7.

Referring now to FIG. 7, the shingled track width w₁ defines the widthof the first written track between the first written edge 702 and thesecond written edge 704. According to one approach, the second writtenedge 704 may be a first written edge of a second written trackoverlapping part of the first written track.

Moreover, the reader width w₂ defines the distance between the reader'souter edges 706, while the shingled reader guard bands w_(3A) and w_(3B)define the distance between the reader's outer edges 706 and the writtenedges 702 and 704 respectively. According to various approaches, thevalues of the shingled reader guard bands w_(3A), w_(3B) may be the sameor different, depending on the position of the reader. The relativeposition of the reader with respect to a given written track may varywith time for a given magnetic tape head due to various factors (e.g.,temperature, humidity, mechanical imperfections, movement of the reader,etc.).

According to one illustrative example, which is by no means meant tolimit the scope of the invention, the shingled track width w₁ may be4.75 μm (microns). Furthermore, the reader width w₂ may be 2.3 μm, whileboth the shingled reader guard bands w_(3A) and w_(3B) may be 1.23 μm(e.g., the reader is centered between the first and second written edgesin a direction perpendicular thereto).

In some approaches, if too much of the reader is positioned over anadjacent written track rather than the track of interest, the reader maynot be able to read the data written on the track of interest. It ispreferred that a shingled reader guard band ensures that 100% of thereader width is within the first and second written edges of a givenshingled track of interest. However, in one approach, a reader may beable to successfully read the data stored in a given written track ofinterest when approximately 10% of the reader width is outside the planeof the first and/or second written edges of the given written track ofinterest. Therefore, a shingled reader guard band may include 10% of thereader width as shown in Equation 2; but could be more or less dependingon the desired embodiment.

Shingled reader guard band=1.23 μm+0.10(2.3 μm)  Equation 2

Thus, with continued reference to the present illustrative example, theshingled reader guard bands may each be 1.46 μm.

Depending on the dimensions and/or conditions for a given magnetic tapehead, a threshold deviation value (e.g., σ_(max)) may be calculated fromthe magnetic tape drive design. In a preferred approach, the thresholddeviation value may incorporate an appropriate stopwrite to filter thedata such that written data may be successfully read back later(explained in further detail below). The threshold deviation value mayvary as to preferably accommodate any possible PES distribution. Thus,when analyzing a given data set of a given data storage system, the datamay be evaluated as a distribution (e.g., a normal distribution).According to various approaches, the deviation value a may incorporate,but is not limited to a factor “M” which may have a value of 1, 2, 3,4.5, etc. or any other value which would be obvious to one skilled inthe art upon reading the present description. In one illustrativeexample, the factor M may have a value of 3 such that the deviationvalue may be represented by 3σ (3σ_(total)) for a distribution of agiven data set's PES. In one approach, the corresponding deviation valuemay be within the shingled reader guard band value calculated above, asshown in Equation 3.

3σ_(total)=1.46 μm  Equation 3

Once the equation is simplified and both sides are divided by 3, theresulting σ_(total) value (e.g., standard deviation) is 0.49 μm.

However, the value σ_(total) includes a combination of the deviations ofboth the written edge (σ_(w)) and the reader edge (σ_(r)) of themagnetic tape head. Equation 4 shows the relationship between σ_(total)and the deviation values (σ_(w) and σ_(r)) of the two signals combinedto form σ_(total).

σ_(total)=(σ_(w) ²+σ_(r) ²)^(1/2)  Equation 4

However, because the tape path and/or the actuator of the magnetic tapehead may not able to distinguish the difference between when the head isreading and when the head is writing in some embodiments, σ_(w) may beconsidered the same value as σ_(r). Therefore, Equation 4 allows foreither the maximum deviation of the written edge or the deviation of thereader edge to be calculated at any time. In one approach, the σ_(w)value may be calculated by simplifying Equation 4 as is shown inEquation 5.

0.49 μm=(σ_(w) ²+σ_(w) ²)^(1/2)  Equation 5

Once simplified, Equation 5 results in the value for σ_(w) as 0.35 μm.Therefore, according to the present illustrative example, a deviation of0.35 μm may be incorporated in various embodiments as a thresholddeviation value (e.g., σ_(max)), including any of the embodimentsdescribed and/or suggested herein.

As noted above, the first value (e.g., σ_(raw)) is compared to thepredetermined threshold (e.g., σ_(max)) in operation 606. With continuedreference to FIG. 6, operation 608 includes determining a stopwritethreshold based on the first value (e.g., σ_(raw)), when the first valueexceeds the predetermined threshold (e.g., σ_(max)).

In one approach, the stopwrite threshold may be determined by selectinga stopwrite value preassociated with the given σ_(raw) value. In apreferred approach, the stopwrite value may be listed in a look up table(LUT) having stopwrite values calculated for various σ_(raw) values, aplot as depicted in FIG. 8, etc. In another approach, the stopwritevalues may be calculated in real-time and then implemented, as thecurrent PES samples are measured.

Referring to FIG. 8, stopwrite values may be calculated for variouspossible σ_(raw) values using a variance formula known in the art. Theseσ_(raw) values (along the x-axis) and their corresponding stopwritevalues (along the y-axis) may be stored in a plot as is shown in FIG. 8for future use. As discussed above, the maximum desired σ_(raw) valuemay preferably be 0.35 μm which corresponds to that which is depicted inthe graph.

With continued reference to FIG. 6, operation 610 includes determiningif the current PES sample is greater than the stopwrite thresholdacquired in operation 608. In the case that the current PES sample is infact greater than the stopwrite threshold, operation 612 of the method600 disables writing.

In a preferred approach, if writing is enabled or disabled during aninterval, it is enabled or disabled only for the current interval. It ispreferred that, at the start of each new interval, the logic may be runto determine if the writing should be enabled or disabled for that giveninterval. In another approach, if writing is enabled or disabled duringan interval, it may remain enabled or disabled for at least one, atleast two intervals, multiple, etc. intervals, regardless of the logic.

With continued reference to FIG. 6, operation 614 includes not updatinga truncated value (e.g., σ_(truncated)). According to a preferredapproach, the truncated value is not updated when writing is disabled.More information about σ_(truncated) is provided below, includingoperations when σ_(truncated) is updated.

Referring back to operation 606, if it is determined that the firstvalue (e.g., σ_(raw)) fails to exceed the predetermined threshold (e.g.,σ_(max)), then a determination is made whether the current PES sample isgreater than a second value (e.g., four times σ_(raw)). See operation616.

According to various approaches, the second value may be calculated byincorporating any formula, preferably one which includes σ_(raw). In apreferred approach, the second value may act as a stopwrite thresholdalthough the σ_(raw) value does not exceed the value of σ_(max).Therefore, rather than having no stopwrite threshold, the writingoperation may be protected against any sudden fluctuations in thecurrent PES sample, which could later cause readback errors. The secondvalue itself can be any value providing the aforementioned result.According to one illustrative example, calculating a second value mayinclude, but is not limited to N×σ_(raw), where N signifies apredetermined value such as 2, 3, 4, 5, etc.

With continued reference to FIG. 6, if it is determined that the currentPES sample is greater than the second value (e.g., four times σ_(raw)),the method 600 precedes to operation 612 and 614 as described above.

However, if it is determined that the current PES sample is not greaterthan the second value (e.g., four times σ_(raw)), the method 600proceeds to operation 618 which enables writing.

Similarly, referring back to operation 610, if the current PES sample isdetermined to not be greater than the stopwrite threshold, then themethod 600 proceeds to operation 618, thereby enabling writing asdescribed above.

With continued reference to FIG. 6, once writing has been enabled inoperation 618, the method 600 proceeds to update a truncated value(e.g., σ_(truncated)) and verify that the truncated value is less thanσ_(max). See operation 620.

According to a preferred approach, a method may include updating atruncated value (e.g., σ_(truncated)) by incorporating the current PESsample when writing is enabled. In one approach, the truncated value maybe a standard deviation or variance of PES samples.

In some approaches, the truncated value may be compared to thepredetermined threshold (e.g., σ_(max)). If the truncated valuemaintains a value at, or below the predetermined threshold, then it maybe expected that, when reading back the written data on the track, noerrors will occur.

According to one approach, if the σ_(raw) is greater than the σ_(max),then the method of truncated normal distributions (e.g., σ_(truncated))may be incorporated to determine the truncation value such that thecorrect number of samples may be eliminated, but the PES values writtento tape have the same standard distribution of σ_(max). Thus, theσ_(raw) of the data which may be written to the tape will preferably beless than the value σ_(max). This may be accomplished by obtaining thecorrect truncation value from a formula, a look up table, apredetermined value, a chart, etc. For example, the SW Threshold line inFIG. 8 may be used. The foregoing feature is an important featureimplemented in some embodiments because it guarantees that data will bewritten to tape with no more σ than the σ_(max), no matter how high theactual σ really is. Although in some embodiments this design maysacrifice capacity by increasing stopwrite frequency, it will preferablyensure that no errors will occur during reading.

For example, if the σ_(truncated) value for the data actually written tothe aforementioned track remains less than σ_(max), there should be noerrors when reading back that same portion of the track. However, if theσ_(truncated) value for the data being written to the aforementionedtrack rises above the σ_(max) value, errors may be expected to occurwhen later reading the data written to that same segment of the track.According to one approach, such errors may be caused by not havingenough of the intended data successfully written to the track; assuggested by the high deviation. Therefore, it may be desirable toperform some additional evaluations in the even that σ_(truncated) isnot less than σ_(max).

Referring now to FIG. 9, a graph displays results from implementation ofone illustrative embodiment, which in no way is meant to limit theinvention. The graph of FIG. 9 depicts the outcome of incorporating amethod similar to and/or the same as that described in method 600, to agiven set of data. As shown, the σ_(raw) and stopwrite threshold (SWThreshold) values change as the PES is evaluated at predeterminedintervals. Moreover, the σ_(truncated) value remains at or below the maxvalue of 0.35 μm for this illustrative example, thereby ensuring thatthe data being written will be able to be successfully read back.

According to various approaches, the geometry of the data storage system(e.g., track width, reader width, etc.), may contribute in determiningthe allowable distribution during writing for various embodiments.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as “logic,” a “circuit,” “module,” or“system.” Furthermore, aspects of the present invention may take theform of a computer program product embodied in one or more computerreadable medium(s) having computer readable program code embodiedthereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a non-transitory computer readable storage medium. Anon-transitory computer readable storage medium may be, for example, butnot limited to, an electronic, magnetic, optical, electromagnetic,infrared, or semiconductor system, apparatus, or device, or any suitablecombination of the foregoing. More specific examples (a non-exhaustivelist) of the non-transitory computer readable storage medium include thefollowing: a portable computer diskette, a hard disk, a random accessmemory (RAM), a read-only memory (ROM), an erasable programmableread-only memory (EPROM or Flash memory), a portable compact discread-only memory (e.g., CD-ROM), a Blu-ray disc read-only memory(BD-ROM), an optical storage device, a magnetic storage device, or anysuitable combination of the foregoing. In the context of this document,a non-transitory computer readable storage medium may be any tangiblemedium that is capable of containing, or storing a program orapplication for use by or in connection with an instruction executionsystem, apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a non-transitory computer readable storage medium and that cancommunicate, propagate, or transport a program for use by or inconnection with an instruction execution system, apparatus, or device,such as an electrical connection having one or more wires, an opticalfibre, etc.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fibre cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer, for example through the Internet using an Internet ServiceProvider (ISP).

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

It will be clear that the various features of the foregoingmethodologies may be combined in any way, creating a plurality ofcombinations from the descriptions presented above.

It will also be clear to one skilled in the art that the methodology ofthe present invention may suitably be embodied in a logic apparatuscomprising logic to perform various steps of the methodology presentedherein, and that such logic may comprise hardware components or firmwarecomponents.

It will be equally clear to one skilled in the art that the logicarrangement in various approaches may suitably be embodied in a logicapparatus comprising logic to perform various steps of the method, andthat such logic may comprise components such as logic gates in, forexample, a programmable logic array. Such a logic arrangement mayfurther be embodied in enabling means or components for temporarily orpermanently establishing logical structures in such an array using, forexample, a virtual hardware descriptor language, which may be storedusing fixed or transmittable carrier media.

It will be appreciated that the methodology described above may alsosuitably be carried out fully or partially in software running on one ormore processors (not shown), and that the software may be provided as acomputer program element carried on any suitable data carrier (also notshown) such as a magnetic or optical computer disc. The channels for thetransmission of data likewise may include storage media of alldescriptions as well as signal carrying media, such as wired or wirelesssignal media.

Embodiments of the present invention may suitably be embodied as acomputer program product for use with a computer system. Such animplementation may comprise a series of computer readable instructionseither fixed on a tangible medium, such as a computer readable medium,for example, diskette, CD-ROM, ROM, or hard disk, or transmittable to acomputer system, via a modem or other interface device, over either atangible medium, including but not limited to optical or analoguecommunications lines, or intangibly using wireless techniques, includingbut not limited to microwave, infrared or other transmission techniques.The series of computer readable instructions embodies all or part of thefunctionality previously described herein.

Those skilled in the art will appreciate that such computer readableinstructions can be written in a number of programming languages for usewith many computer architectures or operating systems. Further, suchinstructions may be stored using any memory technology, present orfuture, including but not limited to, semiconductor, magnetic, oroptical, or transmitted using any communications technology, present orfuture, including but not limited to optical, infrared, or microwave. Itis contemplated that such a computer program product may be distributedas a removable medium with accompanying printed or electronicdocumentation, for example, shrink-wrapped software, pre-loaded with acomputer system, for example, on a system ROM or fixed disk, ordistributed from a server or electronic bulletin board over a network,for example, the Internet or World Wide Web.

Communications components such as input/output or I/O devices (includingbut not limited to keyboards, displays, pointing devices, etc.) can becoupled to the system either directly or through intervening I/Ocontrollers.

Communications components such as buses, interfaces, network adapters,etc. may also be coupled to the system to enable the data processingsystem, e.g., host, to become coupled to other data processing systemsor remote printers or storage devices through intervening private orpublic networks. Modems, cable modem and Ethernet cards are just a fewof the currently available types of network adapters.

It will be further appreciated that embodiments of the present inventionmay be provided in the form of a service deployed on behalf of acustomer to offer service on demand.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of an embodiment of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

What is claimed is:
 1. A computer program product, the computer programproduct comprising a computer readable storage medium having programinstructions embodied therewith, the program instructions executable bya controller to cause the controller to perform a method comprising:update, by the controller, a first value based on a current positionerror signal sample; determine, by the controller, whether the firstvalue exceeds a predetermined threshold; determine, by the controller, astopwrite threshold based on the first value when the first valueexceeds the predetermined threshold; determine, by the controller,whether the current position error signal sample exceeds the stopwritethreshold; disable, by the controller, writing when the current positionerror signal sample exceeds the stopwrite threshold; and enable, by thecontroller, writing when the current position error signal sample doesnot exceed the stopwrite threshold.
 2. A computer program product asrecited in claim 1, comprising: program instructions executable by thecontroller to compare the current position error signal to a secondvalue calculated using the first value when the first value does notexceed the predetermined threshold; program instructions executable bythe controller to disable writing when the current position error signalsample exceeds the second value; and program instructions executable bythe controller to enable writing when the current position error signalsample does not exceed the second value.
 3. A computer program productas recited in claim 1, comprising program instructions executable by thecontroller to update a truncated value using the current position errorsignal value when writing is enabled, wherein the truncated value is notupdated when writing is disabled; and logic for comparing the truncatedvalue to the predetermined threshold.
 4. A computer program product asrecited in claim 1, wherein the head is a magnetic head.
 5. A computerprogram product as recited in claim 1, wherein the first value is astandard deviation or a variance of position error signal samples.
 6. Acomputer program product as recited in claim 1, wherein the stopwritethreshold is determined by selecting a stopwrite value preassociatedwith the first value.
 7. A computer program product as recited in claim1, wherein the controller performs the method at intervals of less than1 millisecond.
 8. A data storage system, comprising: a head; a drivemechanism for passing a medium over the head; a controller electricallycoupled to the magnetic head; and logic integrated with and/orexecutable by the controller, the logic being configured to: update afirst value based on a current position error signal sample; determinewhether the first value exceeds a predetermined threshold; determine astopwrite threshold based on the first value when the first valueexceeds the predetermined threshold; determine whether the currentposition error signal sample exceeds the stopwrite threshold; disablewriting when the current position error signal sample exceeds thestopwrite threshold; and enable writing when the current position errorsignal sample does not exceed the stopwrite threshold.
 9. A system asrecited in claim 8, comprising: logic configured to compare the currentposition error signal to a second value calculated using the first valuewhen the first value does not exceed the predetermined threshold; logicconfigured to disable writing when the current position error signalsample exceeds the second value; and logic configured to enable writingwhen the current position error signal sample does not exceed the secondvalue.
 10. A system as recited in claim 8, comprising logic configuredto update a truncated value using the current position error signalvalue when writing is enabled, wherein the truncated value is notupdated when writing is disabled; and logic for comparing the truncatedvalue to the predetermined threshold.
 11. A system as recited in claim8, wherein the head is a magnetic head.
 12. A system as recited in claim8, wherein the first value is a standard deviation or a variance ofposition error signal samples.
 13. A system as recited in claim 8,wherein the stopwrite threshold is determined by selecting a stopwritevalue preassociated with the first value.
 14. A system as recited inclaim 8, wherein the logic is executed at intervals of less than 1millisecond.
 15. A method, comprising: updating a first value based on acurrent position error signal sample; determining whether the firstvalue exceeds a predetermined threshold; determining a stopwritethreshold based on the first value when the first value exceeds thepredetermined threshold; determining whether the current position errorsignal sample exceeds the stopwrite threshold; disabling writing whenthe current position error signal sample exceeds the stopwritethreshold; and enabling writing when the current position error signalsample does not exceed the stopwrite threshold.
 16. A method as recitedin claim 15, wherein the first value is a standard deviation or avariance of position error signal samples.
 17. A method as recited inclaim 15, comprising: comparing the current position error signal to asecond value calculated using the first value when the first value doesnot exceed the predetermined threshold; disabling writing when thecurrent position error signal sample exceeds the second value; andenabling writing when the current position error signal sample does notexceed the second value.
 18. A method as recited in claim 15, comprisingupdating a truncated value using the current position error signal valuewhen writing is enabled, wherein the truncated value is not updated whenwriting is disabled; and comparing the truncated value to thepredetermined threshold.
 19. A method as recited in claim 15, whereinthe stopwrite threshold is determined by selecting a stopwrite valuepreassociated with the first value.
 20. A method as recited in claim 15,wherein the method is executed at intervals of less than 1 millisecond.